Method and apparatus for automatic alignment of objects and register mark for use therewith

ABSTRACT

A register mark and a system for bringing a pair of register marks into alignment is disclosed. A first register mark comprises a plurality of dots of a first frequency, and a second register mark comprises a plurality of dots of a second, generally higher frequency. When the first and second register marks are overlaid, an interference pattern resulting from the difference in frequency is observed. The first and second register marks are in alignment when the interference pattern produces a maximum bright spot in the center of the register mark. A small relative movement of the first and second register marks results in a larger relative movement of center of the bright spot. The position of the bright spot indicates which direction and how much to move the second register mark in order to achieve image registration. In an alternate embodiment of the present invention, the frequency of dots in each register mark is warped with distance from the center of the mark. The frequency of the dots of the second register mark is also warped with distance from the center, but overall has a generally higher frequency of dots as compared to the first register mark. The interference pattern resulting from an overlay of the latter two register marks results in a bright spot which occurs unambiguously when the two register marks are in alignment. At misalignments of more than one dot width, the bright spot breaks up so that the overlaid register marks may serve as quick visual check of proper alignment.

This is a continuation of application Ser. No. 764,647, filed Sep. 24,1991, now abandoned.

FIELD OF THE INVENTION

The present invention relates to the field of register marks used foradjusting the relative position of two or more objects in order to bringthe objects into alignment.

BACKGROUND OF THE INVENTION

In many manufacturing processes, it is often necessary to align twoobjects, such as two layers in which one layer will overlie the other.For example, in semiconductor manufacturing, successive masks whichdefine the semiconductor structure are placed over the semiconductorsubstrate at various stages in the manufacturing process. Eachsuccessive mask must precisely align with the position of a previousmask on the semiconductor substrate.

As another example, in color printing, an original color image isscanned to separate the original image into several component colors,each of which component color will be separately printed in alignmentwith the layers above and below. Therefore, the color printing platesmust be manufactured so that each of the separate color images areprinted in precise alignment or registration with each other.

Specifically, an original input image is processed to produce fourscreened images: one for each of the printing inks of cyan, magenta,yellow and black. The four screened images are then used to burnprinting plates needed in a four color printing reproduction process. Inburning the plate, the image negative is affixed to a transparentacetate sheet with holes punched. The acetate sheet is then placed onthe plate burning machine, which has metal tabs that fit into the holesin the acetate sheet. Each of the acetate sheets must have negatives atexactly the same position relative to the holes in order to insure thatthe plates have images in the same place. Accurate placement of thenegatives on the acetate sheets is even more important if each plate isto be burned with more than one negative each.

To accurately place the negative on the acetate sheet, register marksare used. The first negative, typically corresponding to the black imageplane, is placed on a light table that has metal tabs to fit the holesin the acetate sheet. The location of the first negative on the firstacetate sheet is not too critical; a typical tolerance is on the orderof 50 mils. The first negative placement determines the general locationof the color image on the printed page.

The next step is to put another acetate sheet over the first sheet, andplace a second negative, say the negative corresponding to the magentaimage plane on the second acetate sheet in precise relation to the firstnegative on the first acetate sheet. To facilitate alignment, imagenegatives have register marks which are not part of the image and arelater removed. The operator examines the register marks, usually withthe aid of a magnifier. By close examination of the register marks, theoperator determines the position of the second negative on the secondacetate sheet for which the two register marks are exactly overlaid. Thesecond acetate sheet carrying the aligned second image negative(magenta) is then removed. The process is repeated for a third acetatesheet and a third negative, say for cyan, and again for yellow. Theacetate sheets are then used as indicated above to burn printing plates.The quality of color printing is dependent upon the accuracy ofalignment of the four color images in the overall plate burning andprinting process.

Automated processes for the automatic alignment of register marks arealso known. The composite image of the two register marks is captured ina memory, the alignment error is measured, and one plate is moved in adirection so as to reduce the measured error. When the measured error isminimized, the register marks are aligned.

There are many known prior art register marks used to align twooverlying transparent layers. One type of conventional register markconsists of crosshairs, i.e. a plus sign. This form of register mark hastwo disadvantages. First, a crosshairs register mark requires closeexamination, usually with the aid of a magnifier, to determine exactalignment. Second, when the two planes are not in good register, it isvery difficult to determine from the nonaligned crosshair compositeimage which direction to move the top plane. Generally, trial and errormovements indicate to the observer which direction to move the topplane. The need for close inspection and trial movements makes automatedadjustment complex and expensive, in both the optical system and in themechanical control system.

SUMMARY OF THE INVENTION

The present invention is embodied in a register mark and system forbringing a pair of register marks into alignment. A first register markcomprises a plurality of dots of a first frequency, and a secondregister mark comprises a plurality of dots of a second, generallyhigher frequency. When the first and second register marks are overlaid,an interference pattern resulting from the difference in frequency isobserved. The first and second register marks are in alignment when theinterference pattern produces a maximum bright spot in the center of theregister mark. A small relative movement of the first and secondregister marks results in a larger relative movement of center of thebright spot.

In an alternate embodiment of the present invention, the frequency ofdots in each register mark is warped with distance from the center ofthe mark. Specifically, the frequency of the dots in the first registermark is higher at the center than at the edges. The second register markalso contains a similar warping in frequency of the dots with distancefrom the center, but overall has a generally higher frequency of dots.The interference pattern resulting from an overlay of the two registermarks results in a bright spot which occurs unambiguously when the tworegister marks are in alignment. At misalignments of more than one dotwidth, the bright spot breaks up so that the overlaid register marks mayserve as quick visual check of proper alignment.

Register marks in accordance with the present invention may also beprinted alongside the color image to check the final alignment of theoverall printing process, or to monitor the operation of a high speedprinting press. Furthermore, register marks in accordance with thepresent invention may be used in projection alignment systems where oneregister mark is printed or otherwise fixed onto a surface, and theother register mark is projected onto the first register mark, as forexample in the manufacture of semiconductor integrated circuits. Ingeneral, as used herein, the term "print" shall mean either thedeposition or removal of material so as to form an image on the surfaceof an object.

DESCRIPTION OF THE FIGURES

FIG. 1 is an illustration of a bottom register mark in accordance withthe present invention.

FIG. 2 is an illustration of a top register mark in accordance with thepresent invention.

FIG. 3 is an illustration of a top and bottom register mark inaccordance with the present invention shown with top and bottom registermarks in alignment.

FIG. 4 is an illustration of a top and bottom register mark inaccordance with the present invention shown with top and bottom registermarks out of alignment by 3 pixels to the right and 2 pixels down.

FIG. 5 is an illustration of a top and bottom register mark inaccordance with the present invention shown with top and bottom registermarks out of alignment by 13 pixels to the right and 5 pixels down.

FIGS. 6A-6K illustrates successive top and bottom register marks inaccordance with the present invention, each successive figure shown withtop and bottom register marks out of alignment by an additional 2 pixelsto the right.

FIG. 7 is a block diagram of an automatic alignment system used inconjunction with a register mark in accordance with the presentinvention.

FIG. 8 is a flow chart in accordance with the present invention for thecomputer program in the CPU of FIG. 7.

DETAILED DESCRIPTION

A register mark in accordance with the present invention is shown inFIG. 1, as a first register mark 10 comprising a plurality of dots of afirst frequency. At least one horizontal line of dots includes dots 14,16 and 18. The frequency at the center of the horizontal line isgreatest at dot 16 near the center of the horizontal line, while thefrequency of dots 14 and 18 decreases generally at distances away fromthe center. Similarly, register mark 10 includes at least one verticalline of dots such as dots 20, 22 and 24. The frequency at the center ofthe vertical line is greatest at dot 22 near the center of thehorizontal line, while the frequency of dots 20 and 24 decreasesgenerally at distances away from the center.

FIG. 2 is a second register mark 12 comprising a plurality of dots. Thearrangement of the second register mark is similar to the first registermark, but the entire pattern is of a generally higher frequency. Forexample, register mark 12 may have 5% higher frequency at all points,which results in a slightly smaller size for the second register mark ascompared to the first register mark.

FIG. 3 shows the first and second register marks in register. When thefirst and second register marks are in alignment, an interferencepattern, produced by the sum and difference of the frequencies of thefirst and second register marks produces a bright spot in the center ofthe register mark. The bright spot results because the dots of theregister mark tend to be coincident near the center, minimizing blackwhile maximizing white area. Due to the frequency difference, the dotsat a distance from the center tend to be more and more non-coincident,resulting in more and more black area. The net effect is a bright spotwith a peak at coordinates defined by column 30 and row 32.

As the first and second register marks are moved out of alignment, thecenter of the bright spot moves away from the center in a direction soas to indicate the direction of the misalignment. Furthermore, if onemark is moved a small amount, the bright spot moves a much largeramount. The ratio is inversely proportional to the difference betweenthe top and bottom register mark frequencies. By using a very smallfrequency difference between the two register marks, a magnification of10 is easy to achieve. An example of an out of register condition isillustrated in FIG. 4 in which the top register mark is moved 3 pixelsto the right and 2 pixels down. The center of the bright spot has movedto a coordinate location defined by column 34 and row 36, a much greatermovement than 3 pixels right and 2 pixels down would indicate.

As the first and second register marks are moved even further out ofalignment, the pattern becomes incoherent, i.e. there is no clearlydefined bright spot. The latter situation is illustrated in FIG. 5 inwhich the top register mark is moved 13 pixels to the right and 5 pixelsdown. The patterns resulting from the use of register marks inaccordance with the present invention are also known as moire patterns,which result from small differences in screen rulings and screen anglesin the printing of color images.

Although dots shown in register marks 10 and 12 are of a generallyrectangular shape, other dot shapes will also work. Furthermore, insteadof frequency warping, one register mark may have a periodic pattern of aconstant single frequency, and the second register mark may have aperiodic pattern of a constant single, slightly higher frequency. Aconstant frequency pattern will produce an appropriate interferencepattern and bright spot to indicate alignment. However, in the case of aconstant frequency pattern, as the bright spot moves quickly to oneside, another bright spot will move in from the other side. When the topand bottom register marks are offset by exactly one dot cycle, thebright spot appears exactly the same as when the top and bottom registermarks are in perfect register. Due to the above described ambiguity, aconstant frequency pattern cannot be used as a quick visual check ofproper alignment, i.e. as a go/no go indicator. The warping of the dotpattern frequency with distance from the center of the register mark,avoids the ambiguity which could result from constant frequency registermarks offset by one dot cycle.

An illustration of the interference patterns resulting from differentrelative horizontal positions of the top and bottom register marks isshown in FIGS. 6A through 6K. In each successive figure, the topregister mark is moved to the right, out of alignment by an additional 2pixels as compared to the prior figure. The bright spot indicatingalignment in FIG. 6A, moves further to the right in FIGS. 6B, 6C, 6D and6E, until substantially disappearing as shown in FIG. 6F. FIG. 6F alsoshows another bright spot moving in from the other side. However, asshown in FIGS. 6G, 6H, 6I, 6J and 6K the new bright spot breaks upbefore it moves to the center of the register mark. It is noted that twopairs of one dimensional register marks may also be used in lieu of thesingle pair of two dimensional register marks shown. In the latter case,one pair of register marks may provide a first bright spot in the formof a bright bar (instead of a circle) to indicate horizontalregistration, while the other pair of register marks provides a secondbright spot in the form of a bright bar to indicate verticalregistration.

A particular advantage of the register mark of the present invention isthat the position of the bright spot indicates which direction and howmuch to move the register mark in order to achieve image registration, afeature particularly useful in automatic alignment systems. FIG. 7 is ablock diagram of a system for automatically aligning two layers 50 and56 using register marks 52 and 54 of the present invention. The systemcomprises an image capture apparatus 40, which may be a video camera ora line of photosensors, an image memory 42, a central processing unit(CPU) 44, and actuators 58 and 60 for moving layer 50 relative to layer56.

In operation, layers 50 and 56 are placed in approximate alignment withregister mark 52 overlying register mark 54. Camera 40 captures an imageof the two register marks for storage in an image memory 42. CPU 44,responsive to the image memory 42 provides a vertical actuation signalon conductor 46 to vertical actuator 58, and a horizontal actuationsignal on conductor 48 to horizontal actuator 60, until register marks52 and 54 are in substantial alignment.

A flow chart for the program of CPU 44 is shown in FIG. 8. First animage of the two overlying register marks is captured at step 62.Thereafter, the peak of the bright spot is determined at step 64. Manytechniques, such as upsampling and interpolation, are known to thoseskilled in the art for determining the position of a peak from a set ofdata points. After the peak is determined, the error between the peak ofthe bright spot and the center of the register mark is calculated atstep 66. If the error is at a minimum, an indication of registration ismade at step 72. However, if the test at 68 indicates an error, thedrive actuators are effected at step 70 to bring the two register marks52, 54 into alignment. A new image is captured at step 62, and theprocess repeats.

Register marks are also often printed alongside the completed colorimage in order to check the overall printing system accuracy. One pairof register marks may be used for each of cyan, magenta and yellow, theregister mark for each color being printed over the register mark forblack. A glance at the pairs of printed register marks will tell whethereach of the color planes of the final image has been accurately aligned.

In the case of a moving printed image, as in a high speed printingpress, the printed register marks may be used to monitor the accuracy ofthe overall printing system. The image of the register marks may becaptured by the use of a line sensor of photosensitive elements to scanthe image of the register marks as the printed register marks move pastthe line sensor. The image processor detects misalignments by theposition of the bright spot and initiates appropriate adjustments oralerts the operator to the status of the error condition. A registermark of the present invention does not require high intensity flashillumination systems or high resolution optics, in order to accuratelydetermine alignment error from the register marks, thereby savingconsiderable cost and complexity in the design of automatic alignmentand monitoring systems.

Finally, register marks in accordance with the present invention may beused in projection alignment systems to align integrated circuit masksused in semiconductor manufacturing. In the latter case, the firstregister mark may be etched or deposited on the substrate, and the otherregister mark projected onto the first register mark. The finishedsemiconductor wafer may contain one or more pairs of overlying registermarks so that an observer may determine at a glance whether each of thesemiconductor masks of the final integrated circuit have been accuratelyaligned.

A program written in QuickBASIC 4.5 to generate a register mark is givenbelow:

    ______________________________________                                        SUB MakeReg (z)                                                               DIM zz(200)                                                                   FOR i = 0 TO 199                                                              za = i - 100                                                                  zb = za * .01                                                                 zc = zb - .2 * zb   3                                                         zd = COS(zc * 50 * z)                                                         zz(i) = zd                                                                    NEXT                                                                          FOR j = 0 TO 199                                                              zj = zz(j)                                                                    FOR i = 0 TO 199                                                              .25 THEN PSET (i, j)                                                          NEXT                                                                          NEXT                                                                          END SUB                                                                       ______________________________________                                    

The z parameter is preferably 1.0 for the first (black plane) registermark 10, and 1.05 for the other register mark 12, corresponding to a 5percent increase in frequency between the first and second registermarks. The above program generates a register mark 200×200 pixels. The-0.25 figure controls the size of the dots in the register mark pattern(i.e. the percent gray, not the frequency), and may be changed fordifferent dot sizes, as desired.

What is claimed is:
 1. A method for aligning first and second objects,wherein said first and second objects are coupled to respective firstand second register marks, said first register mark including a firstplurality of dots at a first frequency and said second register markincluding a second plurality of dots at a second frequency, said methodcomprising:providing said first register mark including a firstplurality of dots at a first frequency arranged in a two dimensionalarray wherein the frequency of dots in said first register mark ismaximum at the center and decreases with radial distance insubstantially all directions from the center of said first registermark; and providing said second register mark including a secondplurality of dots at a second frequency arranged in two dimensionalarray wherein the frequency of dots in said second register mark ismaximum at the center and decreases with radial distance insubstantially all directions from the center of said second registermark, said frequency of dots in said second register mark being greaterthan said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; positioning at least one of said first and second objectsso that said second register mark is disposed in overlaid relation tosaid first register mark, thereby producing a bright spot providing apeak indication having a maximum point substantially common to saidfirst and second overlaid register marks indicative of the registrationof said first and second register marks; detecting the position of saidbright spot providing a peak indication as said maximum point derivedfrom a set of data points; and moving said one of said first and secondobjects to reposition said bright spot providing a peak indicationtowards the center of said one of said first and second register marks.2. A method in accordance with claim 1, wherein said first register markis printed, and second register mark is printed over said first registermark.
 3. A method in accordance with claim 1, wherein said firstregister mark is printed, and second register mark is projected oversaid first register mark.
 4. A method in accordance with claim 1,wherein said first object represents a first color plane of a printedcolor image and said second object represents a second color plane ofsaid printed color image.
 5. A method in accordance with claim 1,wherein said first object is a first semiconductor mask of an integratedcircuit, and said second object is a second semiconductor mask of saidintegrated circuit.
 6. A method for aligning first and second objects,wherein said first and second objects are coupled to respective firstand second register marks, said first register mark including a firstplurality of dots and said second register mark including a secondplurality of dots, wherein the frequency of dots in each register markis minimum at the center and increases with distance in substantiallyopposite directions from the center of each said register mark, saidmethod comprising:providing said first register mark including a firstplurality of dots at a first frequency arranged in a two dimensionalarray wherein the frequency of dots in said first register mark isminimum at the center and increases with radial distance insubstantially all directions from the center of said first registermark; and providing said second register mark including a secondplurality of dots at a second frequency arranged in two dimensionalarray wherein the frequency of dots in said second register mark isminimum at the center and increases with radial distance insubstantially all directions from the center of said second registermark, said frequency of dots in said second register mark being greaterthan said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; positioning at least one of said first and second objectsso that said second register mark is disposed in overlaid relation tosaid first register mark, thereby producing a bright spot providing apeak indication having a maximum point substantially common to saidfirst and second overlaid register marks indicative of the registrationof said first and second register marks; detecting the position of saidbright spot providing a peak indication as said maximum point derivedfrom a set of data points; and moving said one of said first and secondobjects to reposition said bright spot providing a peak indicationtowards the center of said one of said first and second register marks.7. A method for monitoring the alignment of first and second layers,wherein said first and second layers have a predetermined relation torespective first and second register marks, said first register markincluding a first plurality of dots at a first frequency and said secondregister mark including a second plurality of dots at a secondfrequency, said method comprising:providing said first register markincluding a first plurality of dots at a first frequency arranged in atwo dimensional array wherein the frequency of dots in said firstregister mark is maximum at the center and decreases with radialdistance in substantially all directions from the center of said firstregister mark; and providing said second register mark including asecond plurality of dots at a second frequency arranged in twodimensional array wherein the frequency of dots in said second registermark is maximum at the center and decreases with radial distance insubstantially all directions from the center of said second registermark, said frequency of dots in said second register mark being greaterthan said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; positioning at least one of said first and second layersso that said second register mark is disposed in overlaid relation tosaid first register mark, thereby producing a bright spot providing apeak indication having a maximum point substantially common to saidfirst and second overlaid register marks indicative of the registrationof said first and second register marks; detecting the position of saidbright spot providing a peak indication as said maximum point derivedfrom a set of data points; and providing an indication representative ofthe position of said bright spot providing a peak indication relative tothe center of said one of said first and second register marks.
 8. Amethod in accordance with claim 7, wherein said first register mark isprinted, and second register mark is printed over said first registermark.
 9. A method in accordance with claim 7, wherein said first layerrepresents a first color plane of a printed color image and said secondlayer represents a second color plane of said printed color image.
 10. Amethod in accordance with claim 7, wherein said first layer is a firstsemiconductor mask of an integrated circuit, and said second layer is asecond semiconductor mask of said integrated circuit.
 11. A method inaccordance with claim 7, wherein said step of detecting the position ofsaid bright spot indicative of the registration of said first and secondregister marks includessensing one line of said first and secondregister marks; and moving said first and second register marks therebyforming a two dimensional image of said first and second register marks.12. A method for monitoring the alignment of first and second layers,wherein said first and second layers have a predetermined relation torespective first and second register marks, said first register markincluding a first plurality of dots and said second register markincluding a second plurality of dots, wherein the frequency of dots ineach register mark is minimum at the center and increases with distancein substantially opposite directions from the center of each saidregister mark, said method comprising:providing said first register markincluding a first plurality of dots at a first frequency arranged in atwo dimensional array wherein the frequency of dots in said firstregister mark is minimum at the center and increases with radialdistance in substantially all directions from the center of said firstregister mark; and providing said second register mark including asecond plurality of dots at a second frequency arranged in twodimensional array wherein the frequency of dots in said second registermark is minimum at the center and increases with radial distance insubstantially all directions from the center of said second registermark, said frequency of dots in said second register mark being greaterthan said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; positioning at least one of said first and second layersso that said second register mark is disposed in overlaid relation tosaid first register mark, thereby producing a bright spot providing apeak indication having a maximum point substantially common to saidfirst and second overlaid register marks indicative of the registrationof said first and second register marks; detecting the position of saidbright spot providing a peak indication as said maximum point derivedfrom a set of data points; and providing an indication representative ofthe position of said bright spot providing a peak indication relative tothe center of said one of said first and second register marks.
 13. Anapparatus for aligning first and second objects, wherein said first andsecond objects are coupled to respective first and second registermarks, said first register mark including a first plurality of dots at afirst frequency and said second register mark including a secondplurality of dots at a second frequency, said apparatus comprising:saidfirst register mark including said first plurality of dots wherein thefrequency of dots in said first register mark is maximum at the centerand decreases with distance in substantially opposite directions fromthe center of said first register mark; and said second register markincluding said second plurality of dots wherein the frequency of dots insaid second register mark is maximum at the center and decreases withdistance in substantially opposite directions from the center of saidsecond register mark, said frequency of dots in said second registermark being greater than said frequency of said dots in said firstregister mark at corresponding points of said first and second registermarks respectively; means for positioning at least one of said first andsecond objects so that said second register mark is disposed in overlaidrelation to said first register mark, thereby producing a bright spotproviding a peak indication having a maximum point substantially commonto said first and second overlaid register marks indicative of theregistration of said first and second register marks; means fordetecting the position of said bright spot providing a peak indicationas said maximum point derived from a set of data points; and means formoving said one of said first and second objects to reposition saidbright spot providing a peak indication towards the center of said oneof said first and second register marks.
 14. An apparatus in accordancewith claim 13, wherein said first register mark is printed, and secondregister mark is printed over said first register mark.
 15. An apparatusin accordance with claim 13, wherein said first register mark isprinted, and second register mark is projected over said first registermark.
 16. An apparatus in accordance with claim 13, wherein said firstobject represents a first color plane of a printed color image and saidsecond object represents a second color plane of said printed colorimage.
 17. An apparatus in accordance with claim 13, wherein said firstobject is a first semiconductor mask of an integrated circuit, and saidsecond object is a second semiconductor mask of said integrated circuit.18. An apparatus for monitoring the alignment of first and secondlayers, wherein said first and second layers have a predeterminedrelation to respective first and second register marks, said firstregister mark including a first plurality of dots at a first frequencyand said second register mark including a second plurality of dots at asecond frequency, said apparatus comprising:said first register markincluding a first plurality of dots at a first frequency arranged in atwo dimensional array wherein the frequency of dots in said firstregister mark is maximum at the center and decreases with radialdistance in substantially all directions from the center of said firstregister mark; and said second register mark including a secondplurality of dots at a second frequency arranged in two dimensionalarray wherein the frequency of dots in said second register mark ismaximum at the center and decreases with radial distance insubstantially all directions from the center of said second registermark, said frequency of dots in said second register mark greater thansaid frequency of said dots in said first register mark at correspondingpoints of said first and second register marks respectively; means forpositioning at least one of said first and second layers so that saidsecond register mark is disposed in overlaid relation to said firstregister mark, thereby producing a bright spot providing a peakindication having a maximum point substantially common to said first andsecond overlaid register marks indicative of the registration of saidfirst and second register marks; means for detecting the position ofsaid bright spot providing a peak indication as said maximum pointderived from a set of data points; and means for providing an indicationrepresentative of the position of said bright spot providing a peakindication relative to the center of said one of said first and secondregister marks.
 19. An apparatus in accordance with claim 18, whereinsaid first register mark is printed, and second register mark is printedover said first register mark.
 20. An apparatus in accordance with claim18, wherein said first layer represents a first color plane of a printedcolor image and said second layer represents a second color plane ofsaid printed color image.
 21. An apparatus in accordance with claim 18,wherein said first layer is a first semiconductor mask of an integratedcircuit, and said second layer is a second semiconductor mask of saidintegrated circuit.
 22. An apparatus in accordance with claim 18,wherein said means for detecting the position of said bright spotindicative of the registration of said first and second register marksincludesmeans for sensing one line of said first and second registermarks; and means for moving said first and second register marks therebyforming a two dimensional image of said first and second register marks.23. An apparatus in for monitoring the alignment of first and secondlayers, wherein said first and second layers have a predeterminedrelation to respective first and second register marks, said firstregister mark including a first plurality of dots and said secondregister mark including a second plurality of dots, wherein thefrequency of dots in each register mark is minimum at the center andincreases with distance in substantially opposite directions from thecenter of each said register mark, said apparatus comprising:said firstregister mark including a first plurality of dots at a first frequencyarranged in a two dimensional array wherein the frequency of dots insaid first register mark is minimum at the center and increases withradial distance in substantially all directions from the center of saidfirst register mark; and said second register mark including a secondplurality of dots at a second frequency arranged in two dimensionalarray wherein the frequency of dots in said second register mark isminimum at the center and increases with radial distance insubstantially all directions from the center of said second registermark, said frequency of dots in said second register mark being greaterthan said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; means for positioning at least one of said first andsecond layers so that said second register mark is disposed in overlaidrelation to said first register mark, thereby producing a bright spotproviding a peak indication having a maximum point substantially commonto said first and second overlaid register marks indicative of theregistration of said first and second register marks; means fordetecting the position of said bright spot providing a peak indicationas said maximum point derived from a set of data points; and means forproviding an indication representative of the position of said brightspot providing a peak indication relative to the center of said one ofsaid first and second register marks.
 24. A method for aligning firstand second objects, wherein said first and second objects are coupled torespective first and second register marks, said first register markincluding a first plurality of dots at a first frequency and said secondregister mark including a second plurality of dots at a secondfrequency, said method comprising:providing said first register markincluding said first plurality of dots wherein the frequency of dots insaid first register mark is maximum at the center and decreases withdistance in substantially opposite directions from the center of saidfirst register mark; and providing said second register mark including asecond plurality of dots wherein the frequency of dots in said secondregister mark is maximum at the center and decreases with distance insubstantially opposite directions from the center of said secondregister mark, said frequency of dots in said second register mark beinggreater than said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; positioning at least one of said first and second objectsso that said second register mark is disposed in overlaid relation tosaid first register mark, thereby producing a bright spot providing apeak indication having a maximum point substantially common to saidfirst and second overlaid register marks indicative of the registrationof said first and second register marks; detecting the position of saidbright spot providing a peak indication as said maximum point derivedfrom a set of data points; and moving said one of said first and secondobjects to reposition said bright spot providing a peak indicationtowards the center of said one of said first and second register marks.25. A method in accordance with claim 24, wherein said first registermark is printed, and second register mark is printed over said firstregister mark.
 26. A method in accordance with claim 24, wherein saidfirst register mark is printed, and second register mark is projectedover said first register mark.
 27. A method in accordance with claim 24,wherein said first object represents a first color plane of a printedcolor image and said second object represents a second color plane ofsaid printed color image.
 28. A method in accordance with claim 24,wherein said first object is a first semiconductor mask of an integratedcircuit, and said second object is a second semiconductor mask of saidintegrated circuit.
 29. A method for aligning first and second objects,wherein said first and second objects are coupled to respective firstand second register marks, said first register mark including a firstplurality of dots and said second register mark including a secondplurality of dots, said method comprising:providing said first registermark including a first plurality of dots wherein the frequency of dotsin said first register mark is minimum at the center and increases withdistance in substantially opposite directions from the center of saidfirst register mark; and providing said second register mark including asecond plurality of dots wherein the frequency of dots in said secondregister mark is minimum at the center and increases with distance insubstantially opposite directions from the center of said secondregister mark, said frequency of dots in said second register mark beinggreater than said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; positioning at least one of said first and second objectsso that said second register mark is disposed in overlaid relation tosaid first register mark, thereby producing a bright spot providing apeak indication having a maximum point substantially common to saidfirst and second overlaid register marks indicative of the registrationof said first and second register marks; detecting the position of saidbright spot providing a peak indication as said maximum point derivedfrom a set of data points; and moving said one of said first and secondobjects to reposition said bright spot providing a peak indicationtowards the center of said one of said first and second register marks.30. A method for monitoring the alignment of first and second layers,wherein said first and second layers have a predetermined relation torespective first and second register marks, said first register markincluding a first plurality of dots at a first frequency and said secondregister mark including a second plurality of dots at a secondfrequency, said method comprising:providing said first register markincluding a first plurality of dots wherein the frequency of dots insaid first register mark is maximum at the center and decreases withdistance in substantially opposite directions from the center of saidfirst register mark; and providing said second register mark including asecond plurality of dots wherein the frequency of dots in said secondregister mark is maximum at the center and decreases with distance insubstantially opposite directions from the center of said secondregister mark, said frequency of dots in said second register mark beinggreater than said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; positioning at least one of said first and second layersso that said second register mark is disposed in overlaid relation tosaid first register mark, thereby producing a bright spot providing apeak indication having a maximum point substantially common to saidfirst and second overlaid register marks indicative of the registrationof said first and second register marks; detecting the position of saidbright spot providing a peak indication as said maximum point derivedfrom a set of data points; and providing an indication representative ofthe position of said bright spot providing a peak indication relative tothe center of said one of said first and second register marks.
 31. Amethod in accordance with claim 30, wherein said first register mark isprinted, and second register mark is printed over said first registermark.
 32. A method in accordance with claim 30, wherein said first layerrepresents a first color plane of a printed color image and said secondlayer represents a second color plane of said printed color image.
 33. Amethod in accordance with claim 30, wherein said first layer is a firstsemiconductor mask of an integrated circuit, and said second layer is asecond semiconductor mask of said integrated circuit.
 34. A method inaccordance with claim 30, wherein said step of detecting the position ofsaid bright spot indicative of the registration of said first and secondregister marks includessensing one line of said first and secondregister marks; and moving said first and second register marks therebyforming a two dimensional image of said first and second register marks.35. A method for monitoring the alignment of first and second layers,wherein said first and second layers have a predetermined relation torespective first and second register marks, said first register markincluding a first plurality of dots and said second register markincluding a second plurality of dots, said method comprising:providingsaid first register mark including a first plurality of dots wherein thefrequency of dots in said first register mark is minimum at the centerand increases with distance in substantially opposite directions fromthe center of said first register mark; and providing said secondregister mark including a second plurality of dots wherein the frequencyof dots in said second register mark is minimum at the center andincreases with distance in substantially opposite directions from thecenter of said second register mark; positioning at least one of saidfirst and second layers so that said second register mark is disposed inoverlaid relation to said first register mark, thereby producing abright spot providing a peak indication having a maximum pointsubstantially common to said first and second overlaid register marksindicative of the registration of said first and second register marks,said frequency of dots in said second register mark being greater thansaid frequency of said dots in said first register mark at correspondingpoints of said first and second register marks respectively; detectingthe position of said bright spot providing a peak indication as saidmaximum point derived from a set of data points; and providing anindication representative of the position of said bright spot providinga peak indication relative to the center of said one of said first andsecond register marks.
 36. An apparatus for aligning first and secondobjects, wherein said first and second objects are coupled to respectivefirst and second register marks, said first register mark including afirst plurality of dots at a first frequency and said second registermark including a second plurality of dots at a second frequency, saidapparatus comprising:said first register mark including said firstplurality of dots wherein the frequency of dots in said first registermark is maximum at the center and decreases with distance insubstantially opposite directions from the center of said first registermark; and said second register mark including said second plurality ofdots wherein the frequency of dots in said second register mark ismaximum at the center and decreases with distance in substantiallyopposite directions from the center of said second register mark, saidfrequency of dots in said second register mark being greater than saidfrequency of said dots in said first register mark at correspondingpoints of said first and second register marks respectively; means forpositioning at least one of said first and second objects so that saidsecond register mark is disposed in overlaid relation to said firstregister mark, thereby producing a bright spot providing a peakindication having a maximum point substantially common to said first andsecond overlaid register marks indicative of the registration of saidfirst and second register marks; means for detecting the position ofsaid bright spot providing a peak indication as said maximum pointderived from a set of data points; and means for moving said one of saidfirst and second objects to reposition said bright spot providing a peakindication towards the center of said one of said first and secondregister marks.
 37. An apparatus in accordance with claim 36, whereinsaid first register mark is printed, and second register mark is printedover said first register mark.
 38. An apparatus in accordance with claim36, wherein said first register mark is printed, and second registermark is projected over said first register mark.
 39. An apparatus inaccordance with claim 36, wherein said first object represents a firstcolor plane of a printed color image and said second object represents asecond color plane of said printed color image.
 40. An apparatus inaccordance with claim 36, wherein said first object is a firstsemiconductor mask of an integrated circuit, and said second object is asecond semiconductor mask of said integrated circuit.
 41. An apparatusfor aligning first and second objects, wherein said first and secondobjects are coupled to respective first and second register marks, saidfirst register mark including a first plurality of dots and said secondregister mark including a second plurality of dots, said apparatuscomprising:a first register mark including a first plurality of dotswherein the frequency of dots in said first register mark is minimum atthe center and increases with distance in substantially oppositedirections from the center of said first register mark; and a secondregister mark including a second plurality of dots wherein the frequencyof dots in said second register mark is minimum at the center andincreases with distance in substantially opposite directions from thecenter of said second register mark, said frequency of dots in saidsecond register mark being greater than said frequency of said dots insaid first register mark at corresponding points of said first andsecond register marks respectively; means for positioning at least oneof said first and second objects so that said second register mark isdisposed in overlaid relation to said first register mark, therebyproducing a bright spot providing a peak indication having a maximumpoint substantially common to said first and second overlaid registermarks indicative of the registration of said first and second registermarks; means for detecting the position of said bright spot providing apeak indication as said maximum point derived from a set of data points;and means for moving said one of said first and second objects toreposition said bright spot providing a peak indication towards thecenter of said one of said first and second register marks.
 42. Anapparatus for monitoring the alignment of first and second layers,wherein said first and second layers have a predetermined relation torespective first and second register marks, said first register markincluding a first plurality of dots at a first frequency and said secondregister mark including a second plurality of dots at a secondfrequency, said apparatus comprising:said first register mark includinga first plurality of dots wherein the frequency of dots in said firstregister mark is maximum at the center and decreases with distance insubstantially opposite directions from the center of said first registermark; and said second register mark including a second plurality of dotswherein the frequency of dots in said second register mark is maximum atthe center and decreases with distance in substantially oppositedirections from the center of said second register mark, said frequencyof dots in said second register mark being greater than said frequencyof said dots in said first register mark at corresponding points of saidfirst and second register marks respectively; means for positioning atleast one of said first and second layers so that said second registermark is disposed in overlaid relation to said first register mark,thereby producing a bright spot providing a peak indication having amaximum point substantially common to said first and second overlaidregister marks indicative of the registration of said first and secondregister marks; means for detecting the position of said bright spotproviding a peak indication as said maximum point derived from a set ofdata points; and means for providing an indication representative of theposition of said bright spot providing a peak indication relative to thecenter of said one of said first and second register marks.
 43. Anapparatus in accordance with claim 42, wherein said first register markis printed, and second register mark is printed over said first registermark.
 44. An apparatus in accordance with claim 42, wherein said firstlayer represents a first color plane of a printed color image and saidsecond layer represents a second color plane of said printed colorimage.
 45. An apparatus in accordance with claim 42, wherein said firstlayer is a first semiconductor mask of an integrated circuit, and saidsecond layer is a second semiconductor mask of said integrated circuit.46. An apparatus in accordance with claim 42, wherein said means fordetecting the position of said bright spot indicative of theregistration of said first and second register marks includesmeans forsensing one line of said first and second register marks; and means formoving said first and second register marks thereby forming a twodimensional image of said first and second register marks.
 47. Anapparatus in for monitoring the alignment of first and second layers,wherein said first and second layers have a predetermined relation torespective first and second register marks, said first register markincluding a first plurality of dots and said second register markincluding a second plurality of dots, said apparatus comprising:saidfirst register mark including a first plurality of dots wherein thefrequency of dots in said first register mark is minimum at the centerand increases with distance in substantially opposite directions fromthe center of said first register mark; and said second register markincluding a second plurality of dots wherein the frequency of dots insaid second register mark is minimum at the center and increases withdistance in substantially opposite directions from the center of saidsecond register mark, said frequency of dots in said second registermark being greater than said frequency of said dots in said firstregister mark at corresponding points of said first and second registermarks respectively; means for positioning at least one of said first andsecond layers so that said second register mark is disposed in overlaidrelation to said first register mark, thereby producing a bright spotproviding a peak indication having a maximum point substantially commonto said first and second overlaid register marks indicative of theregistration of said first and second register marks; means fordetecting the position of said bright spot providing a peak indicationas said maximum point derived from a set of data points; and means forproviding an indication representative of the position of said brightspot providing a peak indication relative to the center of said one ofsaid first and second register marks.
 48. An apparatus for aligningfirst and second objects, wherein said first and second objects arecoupled to respective first and second register marks, said firstregister mark including a first plurality of dots and said secondregister mark including a second plurality of dots, wherein thefrequency of dots in each register mark is minimum at the center andincreases with distance in substantially opposite directions from thecenter of each said register mark, said apparatus comprising:said firstregister mark including a first plurality of dots at a first frequencyarranged in a two dimensional array wherein the frequency of dots insaid first register mark is minimum at the center and increases withradial distance in substantially all directions from the center of saidfirst register mark; and said second register mark including a secondplurality of dots at a second frequency arranged in two dimensionalarray wherein the frequency of dots in said second register mark isminimum at the center and increases with radial distance insubstantially all directions from the center of said second registermark, said frequency of dots in said second register mark being greaterthan said frequency of said dots in said first register mark atcorresponding points of said first and second register marksrespectively; means for positioning at least one of said first andsecond objects so that said second register mark is disposed in overlaidrelation to said first register mark, thereby producing a bright spotproviding a peak indication having a maximum point substantially commonto said first and second overlaid register marks indicative of theregistration of said first and second register marks; means fordetecting the position of said bright spot providing a peak indicationas said maximum point derived from a set of data points; and means formoving said one of said first and second objects to reposition saidbright spot providing a peak indication towards the center of said oneof said first and second register marks.